This invention relates to a process for the production of oleum having a concentration of 10 to 45% by weight SO.sub.3 and/or sulfuric acid having a concentration of 96 to 100% by weight H.sub.2 SO.sub.4 by burning of sulfur with atmospheric oxygen, reaction of the SO.sub.2 -containing gases to SO.sub.3 -containing gases on vanadium catalysts and absorption of the SO.sub.3 -containing gases for the production of oleum and/or sulfuric acid.
Various processes for the production of oleum and sulfuric acid are known from, and described in detail in, the literature.
In the thermal processes (burning of sulfur with atmospheric oxygen, roasting of pyrites, cleavage of sulfuric acid), nitrogen oxides (NO).sub.x are formed in addition to gases containing sulfur dioxide. These nitrogen oxides are absorbed in the same way as the gases containing sulfur trioxide which are formed from the SO.sub.2 -containing gases on the vanadium catalyst, so that the sulfuric acid formed or the oleum is contaminated with absorbed (NO).sub.x. For many applications, however, the sulfuric acid or the oleum should be free from absorbed (NO).sub.x. In addition, the (NO).sub.x absorbed in oleum or sulfuric acid leads to increased corrosion of steel parts in the sulfuric acid plants. Accordingly, there has been no shortage of attempts to remove the impurities from the sulfuric acid or oleum.
Thus, sulfuric acid free from nitrous vitriol and having a concentration of &gt;80% by weight can be produced by addition of hydrazine compounds at 50 .degree. to 100.degree. C. (DE-OS 1 792 348). Although the process of adding aqueous hydrazine sulfate solutions is basically suitable, it has been found that secondary reactions involving hydrazine sulfate take place at relatively high contents of absorbed (NO).sub.x and merely create a false impression of a reduction in the (NO).sub.x content. In addition, this process is very expensive.
In addition, it is known that monomeric or polymeric compounds containing NH.sub.2 groups are added to remove nitrous acid from waste sulfuric acids (DE-OS 2 831 941). Hydroxylammonium sulfate, urea or amidosulfonic acid are preferably used as suitable compounds. However, this process can only be carried out for sulfuric acids having a concentration of approximately 50 to 90% by weight. In addition, the use of the particularly active amidosulfonic acid, for example, involves high costs. Moreover, considerable effort is involved in the two above-mentioned processes in analyzing the sulfuric acid to monitor its nitrogen content in order to avoid under- or overdosage of the compounds added.
Accordingly, the problem addressed by the present invention was to reduce the content of nitrous gases formed during the burning of sulfur for the production of oleum and/or sulfuric acid and absorbed in oleum and/or sulfuric acid.
This problem has now been solved by the process according to the invention.